No it's false.As for every AVR you can enhance the AtMega328p ADC resolution. See Atmel Application Note : AVR121.

Uno board is for initiation only, she does it well, it was not expected for professionnal use.I don't know the Mega layout but Uno layout is very poor concerning analogical signals.No analogical ground near A0-A5 pins, the layout of Aref line is an horror : I have to sold 100nF capacitor directly on the micro socket to have stable measurment, and so on ............

I'd go with digital over SPI, much faster.Here's a simple chip to start with - 8 channels of 12 bit ADChttp://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en010534http://www.digikey.com/product-detail/en/MCP3208-CI%2FP/MCP3208-CI%2FP-ND/305928